Pipelines are the most efficient way to transport a variety of products such as water, oil and natural gas. A major and growing concern about pipelines is safety and reliability. Parts of existing and future buried pipelines cross areas prone to ground movements which can pose serious hazards to the pipelines. Ground movements are not frequent, but since a usual pipelines failure due to ground movement is rupture, ground movements are considered a high-risk threat to pipelines with potentially catastrophic failure consequences. The catastrophe is due to the high operation pressures and explosive and contaminating nature of the pipeline contents (e.g. hydrocarbon or CO2 pipelines) or the disruption of critical supply in emergency conditions (e.g. water supply for communities after earthquakes).
Ground movements can occur due to variety of sources such as seismic activities, fault slips, downhill creep, landslides, frost heaves and thaw settlements, etc. Many pipelines cross rough terrains, discontinuous permafrost regions, or areas susceptible to seismic activities or heavy rain falls. Considering the vast spread of such areas around the world through which pipelines cross, pipeline exposure to ground movements is a global concern.
When ground movement displaces part of a pipeline, internal forces and moments develop in a certain length of the pipe. It is not practical to design pipelines with a level of strength to resist ground movements, however, pipelines can be designed with a deformability level to deform in such a way that the development of internal forces (associated with the applied displacements) won't reach critical values. Deformability is the ability of pipelines to distribute applied deformations along their length without approaching critical conditions. In other words, pipelines with higher degrees of deformability have a greater capacity to accommodate displacement due to ground movement without excessive strain concentrations.
Based on the direction of the ground movement and the orientation of the pipe axis, a displacement can have three components (with respect to the pipe axis), namely longitudinal, lateral and vertical. For example, frost heave and thaw settlement result in vertical displacements; downhill creep and landslides more likely impose a combination of longitudinal and lateral displacements; and fault slips can cause combinations of longitudinal, lateral and/or vertical displacements. The critical segment of the pipe can go under axial tension and compression, horizontal and vertical shear forces and/or in-plane and out-of-plane bending moments. The magnitudes of these internal forces and moments have an inverse relation with the pipe's ability to move under the ground and distribute the applied displacements to longer distances.
In the case of buried pipelines, the boundary constraints from the backfill and surrounding soil limit the pipe ability to easily move and deform when it is displaced by ground movements. Axial friction between the pipe and surrounding soil resists longitudinal movement; backfill weight resists upward vertical movement; soil vertical bearing resistance prevents vertical downward movement; and passive soil pressure resists lateral movement of a buried pipeline. Continuous pipelines, such as oil and gas pipelines, show high performance under movements causing axial tensile deformation, but, these pipeline's performance level decreases under movements causing lateral and vertical deformations and the pipeline's performance becomes very limited under axial compression. For segmented pipelines such as water supply pipelines, any types of movement causing tensile, compressive or lateral forces and bending deformations can jeopardize the integrity of the pipeline.
There are few engineering solutions in the prior art to modify the boundary conditions of buried pipelines in order to mitigate the risks caused by ground movements. These solutions include using wide trenches with soft back filling soil, installation of polystyrene beads in geo-textile bags around the pipe, or installation of geo-foam cubes on top of the pipe before backfilling the trench.
These methods try to provide soft boundary constraints that allow buried pipes to move when the pipes are displaced by ground movement. However, boundary elements used in these methods are homogenous and essentially have isotropic mechanical behaviour. Therefore, their softness cannot be less than a certain level; otherwise they will be crushed under the weight of the backfill and overburden weight or the soil lateral active pressure. This shortcoming limits the effectiveness of these methods in terms of how much and what types of pipe deformation they can accommodate. An ideal boundary condition is one that resists the soil pressures and—at the same time—accommodates pipe displacements caused by ground movements. This requires a set of elements with an anisotropic mechanical behaviour which are stiff in the direction along which the soil pressure is applied, and is collapsible in the direction along which the pipe moves.